Chemically, the alkaloids are derived from ammonia (NH3) by substituting various organic groups or “compound radicles” (compounds of carbon and hydrogen), for the hydrogen of the ammonia. They are therefore “compound ammonias,” or “amines.” Nitrogen, carbon, and hydrogen, are always present in natural alkaloids, the non-volatile ones, including the greater number, also contain oxygen.
Physiologically, alkaloids as a class have a powerful action on the human and animal frame. The medicinal properties of plants are generally due to these substances, though many are still undiscovered or imperfectly known. They exist in the plant combined with vegetal acids, some of which are characteristic, as aconitic acid in aconite, meconic in opium, igasuric (?) in nux vomica, &c. The very small quantity which may sometimes be fatal (a fraction of a grain of the pure alkaloid), the indefiniteness of many of their chemical reactions, and the facility with which they decompose if too high a heat, or too strong reagents, be employed in their extraction, render the detection often a difficult, and sometimes an impossible matter. Fortunately, however, fresh tests and better processes develop from every case, and other indications, from symptoms and collateral circumstances, rarely fail to bring home the guilt even to the most ingenious and scientific of poisoners.
For extracting the alkaloids from animal matters the following process has been used by the author. Mince finely, digest with rectified spirit and enough acetic acid to just acidify, warm to blood-heat for 15 minutes, filter: this is the first extract. Warm the insoluble matters with more alcohol and filter again: this is the second extract. Repeat the extraction a third time. Keep the three extracts separate. Each should be evaporated at as low a temperature as possible, not exceeding 50° C., and preferably in a vacuum at the ordinary temperature, if this can be done fairly quickly. The syrupy residues must be treated with water and a drop of acetic acid, passed through wet filters to separate fat, rendered just alkaline with ammonia, and shaken with a moderate quantity of a mixture of equal volumes of ether and chloroform (Allen). By a stoppered funnel or burette the ethereal layer is separated, the shaking with ether and chloroform and the separation repeated a second and a third time, the ethereal extracts mixed, transferred to a large porcelain dish, and evaporated, first in a current of air, then in a vacuum or spontaneously. As the solvents evaporate, water generally appears: this hinders any crystallization. Therefore the residue must be rendered dry, then dissolved in a little anhydrous chloroform (dried by standing over fused calcium chloride), and again evaporated in air in a large watch glass. The residue will generally be crystalline under the microscope if any alkaloid be present. Dissolve again in chloroform, transfer to a graduated burette, make up to a convenient volume (say 10 cubic centimetres), and transfer a measured fraction to a number of watch glasses, reserving about one-fourth for any subsequent quantitative test that may be necessary. Allow the liquid in the watch glasses to spontaneously evaporate. To the first add a little water and a very minute quantity of dilute hydrochloric acid, and cautiously taste a portion. A tingling of the lips and subsequent numbness indicate aconite; intense bitterness points to strychnia; if there be no taste at all it is unlikely that any alkaloid is present. There are some alkaloids of a peppery taste; these are irritants, and are not common as poisons. Bitterness is the most frequent characteristic.
2. Moisten the contents of the second watch glass with a little water and a trace of acetic acid, and apply through an incision in the skin of the back of a young frog. He should be kept as comfortable as possible and the symptoms observed. Strychnia readily produces tetanus in this animal; other poisons also have peculiar effects. Some observers have used mice, rabbits, or cats; in the Palmer trial it was observed that dogs were not employed because they were inconvenient and might bite! On the whole this so-called physiological test has been overrated, as it is hardly to be expected that an animal with its back cut and otherwise injured will not exhibit some symptoms; and all who have kept wild animals in confinement will know how soon they become, first almost convulsive from excitement, then finally sink into stupor and die. If necessary, any judge may grant a special licence to the experts in a trial to make experiments on animals, otherwise such cruelty is rendered penal by the Vivisection Act.[3]
3. To the third watch glass, after the contents have been dissolved as before, a drop of a solution of iodine in potassium iodide is added. Nearly all alkaloids give a brown precipitate. If none occur, a negative conclusion may be expected.
4. Test the fourth watch glass in one corner for strychnia by concentrated sulphuric acid and peroxide of manganese; in another corner for morphia by iodic acid and starch; in a third corner for brucia (and morphia) by strong nitric acid. (See the special paragraphs on these reactions, pp. 280, 285.)
5. If there is still no indication, and no information has been obtained from other sources, it may be necessary to employ Dragendorff’s process on the remainder. But if the poison has been discovered, the solution reserved in the burette should be evaporated, dissolved in water and a little dilute acid, avoiding heat, and titrated by Mayer’s reagent to ascertain the quantity.[4]
The second and third extractions of the organs must now be considered. Most of the text-books recommend that all the extracts should be mixed. The objection to this is, that since the alkaloid is usually present in very small amount, the first extraction will remove nearly all of it, while the second and third will mainly contain other matters, and therefore will be only adding to the impurities, and consequently to the difficulty of isolation. If it be worth while, the second and third extracts may be treated separately as above, and should any further quantity of alkaloid be found, it may be determined quantitatively, and the amount added to that already obtained.
It has been proposed to precipitate the original spirituous extract by neutral or basic acetate of lead, which throws down many impurities, but leaves the alkaloids in solution. After filtration, the liquid is treated with a current of sulphuretted hydrogen to remove lead; again filtered, evaporated (as speedily as can be done without overheating) to a moderate bulk, and treated with a little ammonia and with ether-chloroform as before. If the sulphuretted hydrogen be left exposed to the air for some time, it oxidizes to sulphuric acid, which, during and after evaporation, tends to destroy the alkaloid. Hence I have found it advisable to remove the H2S quickly by a current of carbonic acid and warming—previous to evaporation. But this process is not good for alkaloids, as sulphur compounds are often formed, which interfere with subsequent operations.
The foregoing process may fail to extract morphia, curarine, and solanine, as these, being very little soluble in ether-chloroform, may remain behind in the aqueous liquid. This, therefore, should be afterwards treated in one of the following ways:—